In the field of semiconductor devices such as transistors, etc., various bonding methods have been developed for incorporating semiconductor chips in packages and practically used.
In these methods, a technique called die bonding is used to attach a semiconductor chip to a definite position of a substrate for the purposes of ensuring the mechanical connection, electrical connection, heat dissipation, etc. of chips to packages, and is practically performed in the following manners.
(a) Eutectic crystal alloy method: A connection method of scrubbing an Au surface as a conductor surface to which an Si chip is attached with respect to the back surface of the Si chip while heating to about 400.degree. C. to thereby effect alloying by an Au-Si eutectic crystal. However, this method is very expensive due to the use of gold.
(b) Resin bonding method: A method of bonding a chip subjected to a back surface treatment with a pasty adhesive containing a thermosetting resin such as an epoxy resin, etc. and a filler such as Ag, Au, SiO.sub.2, MgO, etc. However, in this method, there is required a long period of time for curing the resin although the mounting work can be performed at ambient temperatures. Also, this method has a disadvantage that the pasty adhesive is inferior in workability because a predetermined amount thereof must be weighed and also the adhesive creates tacky circumstances. Furthermore, such a pasty adhesive has a shortcoming that silver powder is separated by sedimentation during the storage of the paste.
In order to solve the above problems, it has been conceived to provide a die bonding tape comprising an adhesive layer laminated on a release film. More particularly, the die bonding tape is a continuous adhesive tape which comprises an adhesive layer composed mainly of a thermosetting resin and a filler and a release film and which is wound on a reel.
Exemplary as thermosetting resins for the adhesive layer are epoxy resins, polyimide resins, phenolic resins, polyamide resins, silicone resins, etc. as well as modified resins thereof, which form a three-dimensional structure with predetermined amounts of hardeners through crosslinking by heating.
The electroconductive fillers to be added to the resins are powders of metals such as gold, silver, copper, nickel, rhodium, palladium, etc. which pass #100 mesh, preferably #300 mesh. Non-electroconductive fillers include metal oxides such as MgO, Al.sub.2 O.sub.3 and SiO.sub.2. All of these fillers are effective in preventing softening by heat or shrinkage.
In the case of epoxy resins, used as hardeners are acid anhydrides, imidazoles, dicyandiamide, amines, etc. and optionally silane coupling agents, etc.
On the other hand, useful as release films are films and papers release-treated with silicone resins, etc. which include films of polyethylene terephthalate, polypropylene and fluoroplastics. These release-treated films or papers are characterized in that although they are attached to the aforementioned adhesives they are easily removable. The release treatment may be made either to the single side or to both sides of the film or paper, but the treatment to both sides may be preferred.
Practical adhesive tapes for die bonding are about 50 to 100 m long, about 1 to 8 mm wide (this width corresponding to the sizes of semiconductor chips) and about 30 to 150 .mu.m thick, preferably 40 to 100 .mu.m thick. Of the above thickness, the adhesive layer has a thickness of about 10 to 50 .mu.m, preferably 20 to 30 .mu.m, while the release film has a thickness of about 20 to 100 .mu.m, and a release-treated polyethylene terephthalate tape having a thickness of 25 to 75 .mu.m (standard being 38 .mu.m) is usually used.
When winding the above adhesive tapes for die bonding on reels, a preferred winding manner for preventing the inside from contamination is such that the release film faces the outside although either of the adhesive layer and release film may face the outside.
However, no special care has been exercised to the state of the surface of the adhesive layer, and the conventional tapes for die bonding have physical surfaces common to ordinary tapes or films. As a result, the conventional tapes for die bonding have a shortcoming that voids tend to generate in the adherent interface between the surface of the adhesive layer and the die pad when the adhesive layer is bonded to the die pad of a lead frame by pressing a heat rod on the back of the release film by way of heat transfer. And, the voids once formed between the lead frame and the surface of the tape may result in the following troubles:
(1) the voids tend to collect water, solvents, etc. and cause steam explosion by heat stock thereby generating cracks in the lead frame, etc.; and PA1 (2) the bond strength between the lead frame and the surface of the tape for die bonding is insufficient.
Consequently, because of the above-mentioned reasons, the reliability of the semiconductor device decreases.